A spark plug for an internal combustion engine is mounted to an internal combustion engine and is used to ignite air-fuel mixture in a combustion chamber. Generally, a spark plug includes an insulator having an axial hole, a center electrode inserted into a front end portion of the axial hole, a terminal electrode inserted into a rear end portion of the axial hole, a metallic shell provided externally of the outer circumference of the insulator, and a ground electrode provided on the front end surface of the metallic shell and forming a spark discharge gap in cooperation with the center electrode. A resistor is provided in the axial hole between the center electrode and the terminal electrode and is adapted for restraining radio noise generated in association with operation of the engine. The center electrode and the ground electrode are electrically connected to each other via the resistor (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 9-306636).
The resistor is formed through compression and sintering of a resistor composition disposed between the center electrode and the terminal electrode. The resistor composition predominantly contains a conductive material, glass powder, and ceramic particles. In the resistor, the conductive material is disposed in such a manner as to cover the surfaces of particles of glass powder and the surfaces of ceramic particles; as a result, the conductive material forms a large number of conductive paths which electrically connect the two electrodes. A crushed powder of glass is generally used as the glass powder mentioned above.
Meanwhile, in recent years, the operation of an internal combustion engine is controlled in a complicated manner by use of a computer. Thus, in order to more reliably prevent the occurrence of a malfunction of the computer or a like problem, the resistor is required to provide an enhanced effect of restraining radio noise. For enhancement of the effect of restraining radio noise, increasing the resistance of the resistor is effective. However, increasing the resistance is accompanied by a reduction in energy required for spark discharge, potentially resulting in deterioration in ignition performance. Therefore, in order to restrain, to the greatest possible extent, deterioration in energy required for spark discharge while exhibiting a sufficient effect of restraining radio noise, the resistor must have a resistance that falls within a certain relatively narrow range.
However, in the case of using a crushed powder of glass as the glass powder as mentioned above, particles of the crushed powder have greatly different shapes. Accordingly, the arrangement of particles of the glass powder (sintered glass powder) in the resistor formed through sintering may vary greatly among manufactured spark plugs. Therefore, the quantity, thickness, length, etc., of conductive paths formed between particles of the sintered glass powder vary to a relatively great extent, and in turn, the resistance of the resistor may vary greatly among manufactured spark plugs. That is, using the above-mentioned technique encounters great difficulty in more accurately imparting a predetermined resistance to the resistor with restraint of variation in resistance of the resistor. Therefore, in manufacture of spark plugs whose resistance of the resistor falls within a relatively narrow range as mentioned above, yield may deteriorate.
The present invention has been conceived in view of the above circumstances, and an object of the invention is to provide a spark plug for an internal combustion engine which allows a predetermined resistance to be more accurately imparted to a resistor with restraint of variation in resistance of the resistor and in turn, enables enhancement of yield.